Britain has given a back-door bailout worth around £10 billion to the Republic of Ireland in an arrangement that was never explicitly approved by Parliament, it can be revealed.

The money has been pumped into Ulster Bank, a subsidiary of the state-owned Royal Bank of Scotland which was rescued by a public cash injection of £45 billion five years ago.

New figures show that Ulster Bank, which operates predominantly in the Republic despite its name, has accounted for approximately one in every four pounds of losses at RBS since 2008.

The losses in Ireland were not predicted at the time of the bailout and emerged only after the country’s property market collapsed in 2010. After this, some quarterly reports for Ulster — Ireland’s third biggest